Since many years, N,N′-Bis(dialkylaminoalkyl)urea-α,ω-dihalogenalkyl copolymers have found a broad application as additives in electrodeposition baths. These compounds are particularly used as grain refiners in alkaline zinc electrolytes.
The documents U.S. Pat. No. 5,405,523, U.S. Pat. No. 5,435,898 and WO 2004/044269 A2 describe the use of N,N′-Bis(dialkylaminoalkyl)urea-α,ω-dichloro-diethylether copolymers in alkaline zinc or zinc alloy electrolytes.
EP 1 114 206 B1 describes a formulation consisting of N,N′-Bis(dialkylaminoalkyl)urea-α,ω-dihalogenalkyl copolymers and quaternized pyridine-3-carboxylic acids and an aromatic aldehyde, which excels in that the often described formation of bubbles during zinc deposition can be avoided. Comparative experiments, however, show that a formation of bubbles occurs, in some cases after a longer period of time.
Documents WO 2007/747604 A2 and WO 2007/147605 A2 also describe the use of the above-referenced copolymers as additives in combination with quaternized nicotinamide and tripyridinium derivatives in zinc or zinc alloy baths.
Document WO 2007/025606 A1 describes the use of quaternized nitrogen polymers which may be obtained by reaction of a mixture consisting of N-(Dialkylaminoalkyl)-urea and a N,N′-Bis(dialkylaminoalkyl)-urea, or their guanidine or thiourea analogs, respectively, with a dihalogenalkane or epichlorohydrin. The aforementioned additives have found use in the bubble-free deposition of zinc layers from alkaline zinc electrolytes.
The above-mentioned copolymers may be produced according to U.S. Pat. No. 4,157,388 by a 1:1 reaction of a N,N′-bis(dialkylaminoalkyl)urea with a α,ω-dihaloalkane. By addition of a suitable alkylation agent, such as a monohaloalkane or an acid, the polymerization can be terminated and the molecular weight distribution can be set in accordance with the desired application.
Despite the aforementioned possibility of termination, the control of the reaction is extremely difficult with respect to the polymer composition and may lead to product mixtures only reproducible with difficulty. This may lead to, depending on the application, large fluctuations in the process and the product. Thus, it is known that particularly in electrodeposition processes, the molecular weight distribution of polymers does have a significant influence on the performance of the electrolytes with respect to the layer characteristics. Thus, using the above-referenced polymeric additives, as described e.g. in U.S. Pat. No. 5,405,523, U.S. Pat. No. 5,435,898 and WO 2004/044269 A2, zinc layers are often obtained, which tend toward formation of bubbles after a longer period of time. Moreover, the copolymers produced in this way often contain, in addition, an organically bound halogen, which gives, depending on the electrolyte, rise to an AOX contamination. Furthermore, using the production process described in U.S. Pat. No. 4,157,388, no mixed copolymers or oligomers containing different dihalo compounds or diamino compounds in alternating order can be produced.
The documents US 2008/0223726 A1 and EP 1 315 849 B1 describe the manufacture and use of mixed polymers as additives for zinc and zinc alloy electrolytes, which can be produced by a multi-component polymerization. In this case, besides the N,N′-Bis(dialkylaminoalkyl)urea derivatives (in particular urea, thiourea and guanidine derivatives), further amines or polyamines, respectively, or heterocyclic compounds, such as imidazole, are used. In this case, the ratio of the amine component to the halogen component is preferably 1:1. Using the manufacturing process described in both documents, there is often an arbitrary distribution of the reaction components and a formation of polymers having molecular weights which are detrimental to the quality of the deposited metal layer. Depending on the reaction conditions applied, such as reaction times, heating rates, geometries of the reactor, volume-surface-ratios, etc., one may arrive at product compositions reproducible only with difficulties and thus, at additive properties, which have a significant influence on the deposition results. The manufacture of so-called tailor-made polymers and thus, a targeted control of the layer to be deposited is thus complicated.
US 2008/0223729 A1 describes the beneficial influence of N,N′-Bis(dialkylaminoalkyl)guanidine derivatives on the level of brightness of the deposited zinc layers in alkaline zinc electrolytes. The disadvantage of these derivatives is that the higher gloss level is often bought at a worse distribution of the deposition, which represents a significant disadvantage, in particular in the case of drum electrodeposition.
Due to the manufacturing process, the above-mentioned (cationic) copolymers often contain halide ions as counter ions. Using N,N′-Bis(dialkylaminoalkyl)thiourea derivatives, thioronium compounds are often obtained at the polymerization, which have a negative influence on zinc depositions, since particularly these have a poor stability in alkaline mediums and will decompose.
Another disadvantage of the above-mentioned additives and the corresponding electrolytes is the occurrence of strong burnings or burnt deposits in the range of high current densities in case of zinc rich electrolytes (>10 g/L of zinc content).